Inverters are conventionally used to transform energy from direct current sources into alternating currents and to feed them into AC power systems or to loads. The voltage waveform and the phase angle are matched to the alternating current in the power system so that direct current components and phase shifts are minimized. This is important, above all, in the case of public power supply systems, since in this case, precise feed guidelines are predefined by the network operators, with maximum permissible direct current components.
In order to be able to regulate the inverter suitably, zero crossing in the power system is continually detected and is predefined as a zero crossing signal. Additionally, at the output of the inverter, the alternating current is measured as a controlled variable. Inaccuracies resulting from components can arise in the measuring circuits used, which include, for example, shunt resistors or current transformers. These can be, for example, an unwanted offset voltage in current transformers, or a temperature drift. Methods are therefore known from the prior art by means of which measuring circuits are adjusted.
One method consists, for example, in manually adjusting each measuring circuit after its production. Other methods make use of microcontrollers which perform an adjustment procedure after each switching-on of the device.
In measuring circuits which use shunt resistors to detect the current, the losses which occur at the shunt resistors must be taken into account.
According to the prior art, therefore, in the case of inverters of high output, the use of current transformers is provided in order to measure the current at the output of the inverter. With these current transformers, in order to eliminate unwanted offset voltages, methods with manual adjustment during production are used. However, a method is also known wherein a microcontroller integrated into the measuring circuit is used for adjusting the current transformer at every switching-on of the device.
However, with this method, no deviations due to temperature drift or residual magnetism of the current transformer core are rectified. Residual magnetism is caused by changes in the current direction, due to hysteresis effects in the current transformer core. Heat generated during operation, or external temperature changes, lead to temperature drift, which means that inaccuracies caused by temperature-dependent properties of the current transformer components arise in the current transformer.